1. Technical Field
The invention relates generally to image capture systems. More specifically, the invention is related to an improved light sensor and a method of employing the improved light sensor to capture and process image data to improve dynamic range.
2. Background Art
Typical digital image capture systems such as digital cameras employ a sensor, memory and processor. Sensors are manufactured using charge coupled device (CCD) or complimentary metal oxides (CMOS) semiconductor processes. A typical CMOS sensor includes a pixel array having a plurality of pixel cells arranged in rows and columns. A correlated double sampler (CDS), an amplifier, and an analog-to-digital converter are also included for every column of the pixel array. In a typical system, light intensities captured in each pixel cell of the CMOS sensor are directly transferred to a respective CDS. Thereafter, the image data is provided to an amplifier for amplification. The amplified signal is provided to the analog-to-digital converter, which converts the analog signal into a digital signal. Once the image data is converted into digital form, it is often stored in a memory and/or transferred for further processing either locally within the image capture system or remotely within another image processing system, such as a computer.
An important component in the processing of the image data is to compress the data for easier transmission and/or storage. In the prior art approach, each pixel cell generates a pixel value that is first stored in memory. The stored pixel values are then manipulated and compressed by a digital signal processing (DSP) program executing on a processor.
As the size of the pixel arrays grows, the amount of local memory required to store the pixel values also increases correspondingly. For example, as the pixel array grows from 512xc3x97512 to 1024xc3x971024 and assuming one byte per pixel, the minimal memory requirements increase from 262,144 bytes to 1,048,576 bytes for each image captured. Consequently, four times more memory is needed to store a picture.
An additional concern for video image data is the need to meet bandwidth requirements. As an array grows, additional bandwidth is needed to transfer the additional image data at a given frame rate to other image processing systems, such as computers.
As a pixel array grows from 512xc3x97512 to 1024xc3x971024, the bandwidth required increases by approximately four times. For example, assuming 30 frames per second, the uncompressed data rate from a sensor 512xc3x97512 array with (1 byte) per pixel is approximately 7.8 megabytes per second for the 512xc3x97512 array. The data rate is increased to 31.5 megabytes per second for the 1024xc3x971024 array.
To improve picture quality and to add color to a picture, typical image processing systems increase the number of bytes employed to represent each pixel. For color applications, each pixel is typically represented by more than one byte. For example, some color processing systems employ two bytes, or 16 bits, to represent a single pixel value. As each pixel is represented by a greater number of bytes, the memory requirements and bandwidth requirements increase accordingly.
In addition, for increased dynamic range in the image sensor, some current systems propose that additional memory elements be used to hold multiple readings of a single pixel such that two integration times are captured and analyzed. Consequently, it would be desirable to provide an improved sensor that would reduce the memory storage and bandwidth requirements for image capture and transmission.
In an image capture system having a sensor with a first node and a second node, a method of capturing image data is disclosed. The first node is reset. The second node is reset. An image is collected on the first node. A first pixel value is then transferred from the first node to the second node after a first predetermined time period. The first pixel value and a second pixel value is then provided after a second predetermined time period. An arithmetic operation is performed on the first pixel value and the second pixel value. A system for performing the method is also disclosed.